As a wearable device such as a google glass, a smart watch, a fitness band, and the like is emerging, a technique for related elements becomes more important.
A battery or a rechargeable battery employed in the wearable device is restricted in a shape or a size for a design of the wearable device and is required to operate the device for a long time when it is once installed or charged. Accordingly, an energy storage device for the wearable device is required to maximize an energy density for being capable of storing a large amount of energy in a confined space and is also required to have extensity and flexibility for being employed in the wearable device.
Most conventional techniques for making a battery flexible are based on simply coating a battery active material on a flexible substrate or connecting cells including an active material by using extensible connectors. For example, “rechargeable battery having non-metallic current collector and method of manufacturing the same” (Korean Patent No. 10-0714128; published on May 2, 2007) discloses a technique of coating a positive electrode layer and a negative electrode layer on a polymer film having excellent bendability.
However, such methods may cause a bonding problem between a flexible substrate and an active material layer when it is contracted or elongated due to an elongation difference between the substrate and a battery active material. Further, when a thickness of the active material layer is reduced to secure flexibility, the energy density thereof decreases, and when the thickness is increased, the flexibility decreases, so the active material layer may be broken. Furthermore, in the case of employing the extensible connectors which connect the cells, a problem that the energy density deteriorates may arise because the active material may not be charged as much as spaces occupied by the extensible connectors.